A major challenge for producing crude oil in deep waters is the result of severe instability resulting from what is known as slugging.
This phenomenon varies during the lifetime of the platform, since it depends on a number of factors such as the pressure in the reservoir and lines, the size of the lines, the composition of the multiphase fluid (water content), and the aperture of the well's top-side choke valve, among others.
The use of advanced control techniques to eliminate or minimize slugging has great potential for increasing the output of these units, as well as for making the entire process more stable, minimizing product flaring and unscheduled stops.
A simplified explanation of slugging is the following:—since the platforms are located kilometers away from the wells, as the fluid flows through the pipelines, it can give rise to a phenomenon known as slugging, with periods during which fluids accumulate in the risers and only a limited amount reaches the platform, and other periods in which large amounts of fluid (slugging) reach the platform.
The submerged pipelines, particularly those commonly referred to as flow lines and, in particular, those pipelines that run from the production wells to the loading sites can be susceptible to accumulation or retention of material that produces the slugging phenomenon when it is released.
As a rule, said flow lines extend for thousands of meters along the ocean floor, making the job of identifying and controlling these production intermittences even more critical.
Worsening the problem, often the flow line is several meters below the waterline, and a vertical leg or bypass may be required, along with other measures to re-establish suitable flow to the surface.
These problems are worsened when the production lines have descending sections and the top-side facilities do not have large capacitance (or volume) to absorb such disturbances, which may lead to a large number of unscheduled stops due to excessively large volumes in the separators.
Another loss point is that these oscillations result in a mean flow that is less than the potential flow of the lines.
When there is slugging during production, the bottom hole pressure is found to vary from high (due to accumulated liquid—greater loss of hydrostatic pressure and load) to low.
Thus, the flow also varies, with the mean flow possibly being considerably less than the potential flow. For this reason, developing advanced automated control systems to avoid slugging could result in considerable gains for platform operation.
The main object of the present invention is to provide a system that automatically controls and ensures the operation and production in deepwater petroleum wells without the incidence of slugging, by using pressure gauges in numerous alternative points of the flow line, and continuously acting on the production choke valves using aggregate computer algorithms that monitor a set of operating variables.
As advantages for the system of the present invention we have fewer losses due to flaring and fewer unscheduled production stops; the production is optimized by reducing well and processing plant start-up and stopping times due to unexpected slugging in the production wells, and the transients generated in the procedures required to return to production are also optimized; the result is the unit stabilization and an increased equipment lifetime and safety.
These and other advantages are achieved as the system of the present invention “automates” the operational best practices.
Traditionally, there is a paradigm in the prior art that so long as the choke valve on the top-side of the well is kept in the full-open position, the well will be producing at the maximum output.
However, this is not always true, especially in the face of instabilities such as those discussed above.
For instance Hu, B. (2004) in “Characterizing gas-lift instabilities”, Department of Petroleum Engineering and Applied Geophysics, Norwegian University of Science and Technology, Ph.D. Thesis, NTNU, 2004, shows that a control system acting on the top-side choke valve stabilized the process and increased output by 17%. Also consider that, in this case, the mean bottom hole pressure is lower.
The literature has numerous other examples showing a potential gain in output from using different types of technologies aimed to control slugging in petroleum production wells.
Brazilian patent PI9913875-1, filed on Sep. 10, 1999, and its corresponding U.S. Pat. No. 6,041,803 filed on Mar. 28, 2000 describe a method to eliminate severe slugging in multiphase flow lines, and an associated device located near to the point of junction of the flow line, introducing a pressure drop into the flow. However, this document does not consider controlling the choke valve on the wet tree or upstream of the choke valve to control and reduce slugging.
Brazilian document PI0518401-0 describes another prior art attempt to control slugging, and concerns a method and device to control the formation of liquid or gas accumulations in the flow lines by injecting a surface tension reducing agent such as a foamant. A control unit is mentioned, but the aim of said control unit is to control the device, injecting the surface tension reducing agent. Likewise, that document does not include the pressure control proposed in the present invention by monitoring the choke valve on the wet tree or upstream from the choke valve, and the use of computer algorithms to monitor a set of operating variables.
In his Ph.D. Thesis entitled “Severe Slugging Elimination in Ultra Deep Water Tiebacks and Risers” University of Tulsa—College of Engineering and Natural Sciences—Petroleum Engineering, published on Oct. 7, 2002, Jarl Oystein Tengesdal finds that by creating a small additional pressure drop using a choke valve one can eliminate severe slugging. Tests are conducted considering flow points totally or partially obstructed by severe slugging.
As seen, the type of flow known as slugging in the fluids produced in producing oil wells creates major disturbances for production platforms, and causes major variation in process pressures and levels.
In many cases, these disturbances lead to unscheduled production stops, and in others may damage equipment, such as heat exchangers.
Thus, slugging have a major impact on the reliability of the operation, as they affect product availability and quality (BSW, TOG and Moisture).
Therefore, there exists a complex control problem, which is to use measurements to dynamically act on the production choke valves to generate a new type of flow, without major slugging.
Adopting the system of the present invention will allow platforms to be more stable and consequently increasing their output by implementing an advanced anti-slugging control system.
This technology will also impact production, minimizing losses with fewer unscheduled stops.